An electrochemical membrane is a semipermeable or permeable membrane that provides ionic transport across the membrane. The ionic transport may be through diffusion, migration or convection, and occurs in response to a difference in chemical concentrations across the membrane or electrical polarization in devices such as a battery separator, a fuel cell membrane, a flow cell membrane, and the like. An electrochemical membrane may be used as an ion exchange device, for example, in a water softener system. Electrochemical membranes may also be used as ion selective membranes, for example, in a sensor such as a fluorine ion sensor.
Batteries, such as lithium ion batteries, conventionally contain a liquid electrolyte, such as an organic carbonate-based electrolyte, used in conjunction with a porous polymer membrane. Organic liquid electrolytes have a disadvantageous property that they may present a risk of a thermal event if not properly handled. Safer alternatives to a liquid electrolyte include a non-porous solid or polymer electrolyte. A polymer electrolyte has flexibility, but typically has an ionic conductivity that is too low for use in electrochemical applications. A solid electrolyte, which includes an inorganic solid electrolyte (ISE) material, has a sufficiently high ionic conductivity for use in electrochemical applications, but is rigid or inflexible. Various ISEs demonstrate comparable ionic conductivity to current liquid electrolytes, have resistance to thermal events, and are structurally rigid or inflexible which prevents penetration and possible short circuiting from Li metal dendrite growth. The benefits of an ISE are typically realized only in a purely solid state battery (SSB) where the ISE is a dense, sintered plate, and the plates are layered or stacked within the battery. Volume production of this battery configuration is difficult, for example, for an automotive battery. Another limitation to an ISE battery is that any active material must be in close physical contact with the ISE to allow for ionic transfer. For a dense sintered ISE plate, this may limit their application to thin layers of active material deposited directly onto the surface of the electrolyte. For these reasons, inorganic solid electrolytes are presently only being used in thin film batteries, where cathode, solid electrolyte and anode are deposited layer by layer by vapor deposition techniques such as sputtering.
A battery having a flexible membrane allows for high volume production that can be incorporated into a wound cell, e.g. spiral shaped cell, from a continuous roll. Recently, a flexible composite membrane cast from a random dispersion of ISE particles encapsulated into a polymer solution was used in conjunction with a conventional liquid electrolyte, thereby allowing for the use of ISE material with a flexible membrane and a wound cell. This membrane typically has too low of an ionic conductivity for electrochemical applications such as batteries, likely because of high interfacial resistance between the inorganic particles and polymer matrix.
Therefore, a need exists for an electrochemical membrane having an ISE that is both flexible and has high ionic conductivity.